Background:
There are two different analog circuits that are named ringmodulators on synths. One is the real one made with transformers and germanium diodes. It has been around for a very long time and has been used e.g. in SSB radio. This diode design was used in the fifties and early sixties by a.o. Stockhausen. The other design is a transistors-only circuit and is officially named a balanced modulator or four-quadrant multiplier. This is the circuit that is used in virtually all analog synths.

The diode ringmodulator however sounds quite different to the later balanced modulator. The difference is caused by the heavy harmonic distortion in the original diode/transformer design. This patch building block is capable of the sound of both analog types plus a vast range of other 'in between' settings. Additionally it can serve as a harmonic distortion unit with the choice of only odd harmonic distortion or a controllable mix of odd and even harmonics.

Operation:
There are five knobs/controls to set the working mode and all five are in essence controllable by a control signal or another audio signal to create even more possibilities. Just tweak to your liking.

How it works:
Like the balanced modulator there is a ladder of two multipliers biased by a bias signal. The bias signal value is by default +64 units. This bias goes into one of the inputs of a multiplier (A) and the output of the multiplier goes into one of the inputs of a second multiplier (B). The other inputs of the two multipliers are used as the two signal inputs, also named A and B. There are two points where output is taken from, after the first and after the second multiplier. A crossfader fades between these two outputs to fade between AM and RM mode.
On output A is basically the input signal of A multiplied by the bias signal. On output B is the product of A and B multiplied by the bias signal.

The whole trick is in the extra circuits that create the harmonic distortion. This harmonic distortion is made by taking the quadrature of a mix of the input and output signals, and subtract this from the bias signal for the ladder (done in the LevAdd module). This will create odd harmonic distortion. To the quadrature signal is also added a controllable amount of the signal before it is quadrated, which will add the even harmonics.

When both inputs receive a sine wave and the distortion signal that is subtracted from the bias signal comes from the output of multiplier B (this output signal is named C) the sound of the diode ringmodulator is created. But when there is no feedback to the bias, the clean sound of the balanced modulator is heard. The diode ringmodulator sound gets its characteristic sound because the extra harmonics caused by the harmonic distortion get 'ringmodulated' as well. Meaning that not only the sum and difference frequencies of the partials in the input signal are created but also the harmonics of these sum and difference frequencies. Which are not produced by the balanced modulator.

The signal that is subtracted from the bias value is a mix of the input signals A and B and the output signal C. This allows for a whole range of different characters in the final sound. E.g. if the distortion control signal is taken from input A and the output signal is the signal from multiplier A, the circuit will work as a straight ahead harmonic distorter where the amount of distortion and the odd/even balance can be set. For this the Mode knob needs to be set to AM and the Type knobs to A and A|B.
If a signal is present at the B input and on e or both of the Type knobs is slightly opened, a subtle ringmodulation effect is added to the distortion. This is a nice setting when B gets a sinewave signal around 8kHz and e.g. a drumloop is fed to the A input. This gives an effect which sounds a bit like digitization, but is controllable in intensity and character and sound a bit more eerie. When the second Type knob, that fades between A|B and C, is now opened the effect will get strongly like the fifties ringmodulator sound, especially with high distortion depth.
Well, just play with the assigned knobs and discover yourself.

A slight touch of chorus is often the icing on the ringmodulator cake.

To emulate the behaviour of the AC coupled inputs of the analog circuits two highpass filters set to relatively low cutoff frequencies are used on the inputs. The input mixers also act as integrators (read: shallow lowpass filters) set to limit the bandwidth. Cutoff is around 4kHz. This emulates the effect when a LFO pulsewave is fed to one input while the other input receives an audio frequency signal. This is a vintage way to get rhythmic pulses, a very popular trick on e.g. the EMS Synthi. On every flank of the pulse wave the circuit will pass a short blip to the output, an asymmetric pulse gives swing to the pulse.

There are two technical issues when using feedback techniques like used here to control the bias to get the harmonic distortion: 1) this type of feedback has a tendency to oscillate at half the sample rate and 2) when the feedback is nonlinear (like here) the circuit is potentially chaotic.
In this actual circuit chaotic behaviour is unlikely, as it is operated outside the potentially chaotic range. But the tendency to oscillate at half the sample rate is a clear and present danger. This little devil is conveniently taken care of by using an integrating mixer set to a cutoff of about 4kHz in the feedback path to the bias signal. This mixer also conveniently inverts the quadrature of the output signal used for the feedback and limits the total feedback to under unity gain to keep it out of the chaotic range. So, in normal circumstances the circuit is pretty stable. Just don't change anything on the modules that have no name, as that is a code that the module should be left just like it is.

(Through this publication here I claim prior art to this particular design. Its baptized the "Klangmodulator". If anyone ever saw or knows of a similar design, let me know, as then I will drop my claim.)

Have fun,
/Rob

KlangModulator.pch2

Description:

A versatile 'ringmodulator' building block that is capable of emulating both the traditional diode ringmodulator and the transistorised balanced modulator. Plus some more. Additionally it can be used as a harmonic distortion circuit capable of both odd ha

This is a great post, Rob. Thanks. When I started out in electronic music there weren't very many synths or modules we could afford so the DIY movement was really big. One of the easiest things to build was on of these transformer/diode ring modulators. (Finding a good sine wave oscillator to use with it was a bigger challenge actually.) Anyhow, these were a lot of fun. We used them mainly for processing acoustic instruments with pickups.

Later, when electronic multipliers became available, I noticed there was a difference in the sounds. For one, you could almost completely null out the modulator oscillator which was very hard to do with the transformer/diode designs. The new circuits sounded - more electronic.

For a while I ran a small business making electronic ring modulators using a laser trimmed multiplier IC from Analog Devices. It had excellent modular rejection and sounded great. I never thought much of going back to the transformer/diode sound. (Funny, in those days we were always looking for the sounds of the future - today it seems we are lusting after the "vintage" sounds of the past.) This patch does it all. I'm very impressed.

I'd like to see you, Rob, take this into the frequency shifter realm. The ring modulators generate what are called the sum and difference frequencies, or the upper and lower sidebands. For example, given 2 sine wave inputs at as 1000 and 100 Hz, the ring modulator will produce resulting waves of 1100 and 900 Hz. Of course, when you use non-sine wave with lost of harmonics, the resulting output is very complex and rich, but these sum and difference relationships are maintained for each of the harmonics in the inputs. By contrast, the frequency shifter provides only the sum or difference (upper or lower sideband). It's a different effect, but in some cases it's more useful musically.

I make these comments as food for thought. This is superb work and very enlightening.

I'd like to see you, Rob, take this into the frequency shifter realm. The ring modulators generate what are called the sum and difference frequencies, or the upper and lower sidebands. For example, given 2 sine wave inputs at as 1000 and 100 Hz, the ring modulator will produce resulting waves of 1100 and 900 Hz. Of course, when you use non-sine wave with lost of harmonics, the resulting output is very complex and rich, but these sum and difference relationships are maintained for each of the harmonics in the inputs. By contrast, the frequency shifter provides only the sum or difference (upper or lower sideband). It's a different effect, but in some cases it's more useful musically.

In my experience it suffices to add some harmonic distortion on both of the frequency shifter outputs (and then add the input signal to both output signals to get a stereo signal). Otherwise it wobbles pretty much and sounds a bit overtdone, well, to my ears.
In the museum depot (where Wout still has the key to) there is a really old Bode shifter by Moog. Should look one day if it has transformer/diode ringmods in it or not, and test the sound. It may be worthwhile.

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